In each of most vehicles, there is provided a vacuum booster for assisting an operating force that is to be applied to a brake operating member of the vehicle. It is common that the vacuum booster has a negative pressure chamber and a variable pressure chamber that is to be selectively brought into communication with the negative pressure chamber and the atmosphere, such that a communication between the variable pressure chamber and the negative pressure chamber is established when the operating force is not being applied to the brake operating member from an operator of the vehicle, and such that a communication between the variable pressure chamber and the atmosphere is established while the communication between the variable pressure chamber and the negative pressure is being cut off, when the operating force is being applied to the brake operating member, whereby the operating force applied to the brake operating member is assisted by an assisting force generated based on a difference between a pressure in the negative pressure chamber and a pressure in the variable pressure chamber. In this arrangement, the assisting force generated by the vacuum booster and assisting the operating force is increased as long as the difference is being increased. However, when the difference becomes not to be increased, the assisting force becomes not to be increased so that a limit of the assistance made by the vacuum booster is reached. When the limit of the assistance is thus reached, the assisting force is not increased whereby a discomfort could be given to a vehicle operator who operates the brake operating member. Patent literatures identified below disclose techniques each relating to a device capable of recognizing a limit of assistance made by the vacuum booster so as to monitor whether the assistance limit has been reached or not.